1. Field of the Invention
The present invention relates to a lens barrel made of synthetic resin having a helicoid (thread) and a method for producing a molding die assembly for the same.
2. Description of the Related Art
A helicoid of a lens barrel is formed by providing adjacent trapezoidal threads and grooves alternately formed along a circumferential direction of the lens barrel. In the case of a metric thread, the angles of the trapezoidal threads and grooves in a section which includes an axis of the lens barrel are identical. In a developed view, regardless of whether the threads or grooves of the helicoid are provided externally or internally, they are defined by an identical angle. However, in practice, the angle of the trapezoidal grooves is larger than the angle of the trapezoidal threads in a section perpendicular to the direction of an extension of the threads. As the lead angle increases, for example, the lead angle exceeds 30 degrees, the difference in the angle (angle difference) between the trapezoidal grooves and the threads also increases. The reason for this is that the helicoid formed by the threads and grooves, which are defined by an identical angle in a developed view, is wound onto a cylindrical lens barrel. The angle difference occurs because the threads and grooves are defined by an identical angle.
In a conventional helicoid having a small lead angle, the engagement error (meshing error) due to the angle difference between the trapezoidal threads and grooves is negligible. However, if the lead angle is large, the engagement error is not negligible because the helicoids do not come into surface contact, but rather come into line contact due to the meshing error.
In theory, the shape of the trapezoidal threads or the trapezoidal grooves in a section perpendicular to an extension direction of the threads can be determined when the shape of the trapezoidal grooves or the trapezoidal threads is determined. However, it is difficult to precisely analyze the shape of the trapezoidal threads and grooves. This difficulty increases as the lead angle increases. Consequently, it is extremely difficult to produce molding dies used to form a member having a helicoid.